NEC-LIST: When is the height equal to free space ?

Dear list,

I am interested in aircraft antennas and their performance as a function of
height.

I have used NEC to investigate the performance of these antennas at various
heights above ground and in free space.

I had expected that when the antenna was a certain number of wavelengths
above the ground then the antennas would approach the free space
performance but this does not seem to be the case. I have been
concentrating on horizontal antennas.

I went back to basics and moved a dipole to a great height using the GM
card. The theta plot looked ragged and non-physical.

I also looked at two-ray models and other analytic work but these didn't
seem to cover the range from ground to free space.

I'd appreciate you help, guidance and any references.

Regards,

Michael.

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